Frequency control apparatus



Jan. 12, 1954 E. O. KEIZER FREQUENCY CONTROL APPARATUS l Filed sept. 19, 195o 5 sneefs-sneet 1 F' /l .Z/q y Z uw; f

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Elga- "'ef in @my ATTO RN EY Jan. 12, 1954 E. o. KEIZER 2,666,182

FREQUENCY-CONTROL APPARATUS Filed Sept. 19, 1950 3 Sheets-Sheet 2 j@ Ilya 24 Jv; fn f M4- /J ilN 4 50a/war I] aww/fa W -w mf. m65 j??? i /4 INVENTQR ATTORNEY Jan. 12, 1954 u E, 0, KElZER 2,666,182

FREQUENCY CONTROL APPARATUS Filed Sept. 19 1950 5 Sheets-Sheet S5 v INVENTOR i ATTQRNEY' Patented Jan. 12, 1954 FREQUENCY 'CQNTROL APPARATUS Eugene O. Keizer, llrinceton, ^N. J., assignor'to `Radio -Corporationof America, a corporation oi Delaware Application September 19, 1950,5Serial N o. `185,627

Claims.

Thisfinvention-relatesto apparatus -for controlling the irequency of an oscillator in responseto a desired signal.

The frequency of `an oscillator hasbeen generally controlled by a reactance tube placed in parallel with atleast -a portion of the resonant circuit of the oscillator. Aportion of the output oi the oscillator is fe'd back t'o the control grid of the reactancetubeat a9()o phase angle with respect Ito fthe "voltage appearing across `the resonant circuit associated with the oscillator. Thus, the reactance tube may draw either leadingvcr laggingcurrent andact asia capacitance orrinductance placedin parallel with` the resonant circuit. Variationsiin the magnitude ci the 'reactive currentdrawn bythe `reantance tube are controlled in accordance 4with-any 'desired signal.

In the apparatus-of the prior art, hovvever, the feedback from'the oscillator output Itothe grid of the reactance 'tub'elis generally affected byla phase splitting-network. Thevoltage applied to the grid of the reactance tube in such arrangements is generally relatively small Idue tothe voltage divider action oi`the=phase splitter. This impairs the sensitivity of the `device to 4such an extent that in normal `arrangements requiring high sensitivity'the-reactance Atube must'have considerably more gain than the oscillator "tube itself.

It is `accordingly fanobject of Lthe present inventionlto providea meansfor controllingthe irequency of Aan oscillator with `a vmaximum -sensitvity.

It isanother object of this finventionto -provide an improved means and method whereby the voltage which is Vfedfback'fron'ian oscillator to a reactance `tube `associated with the `oscillator -is derivedin such mannerthat the frequency control eifected `by the reactance tube has a maximum sensitivity.

ln the frequency control circuits of the prior art wherein the gain oi a reactance tube yis changedto e'iect a change inthe oscillator frequency, the current drawn by thereactance tube generally has a slightly resistive component. The magnitude of this resistive component varies with the change in gain of thereactance tube produced by the f desired `controlsignals and `therefore `introduces fan amplitude modulation fin thel output of the oscillator thatiis a ,function of the @control voltage appliedfto the reactance tube.

Itis accordingly another object of this invention tovprovidenovel means and apparatus Wherebythe irequencylofanoscllatorzmaybe control 2 led in a simple manner without affectingitsamplitude.

Briey, the 4objectives noted above can be achieved'in accordance with the principles of this invention wherein the voltage applied to control electrode of fthereactance tube is derived from a reactance in series with at least a portion of thel plate current drawn by the oscillator through the normal oscillator circuits. It `will become apparent from a detailed discussion to ollowthat this not onlyfpermitsfthe application of a relatively greater voltageto the reactance tube so as to increase the sensitivity of itscontrol, but `also thatthis arrangementrnakes itipossible to carefully eliminate all resistive components of the current drawnbytthe reactance tube and therefore eliminate any amplitude variations in the output of `the oscillator'that would otherwise be caused by changeslin .therfrequency control voltage.

The Wayin `which the above objects and advantages maybezeiected in accordance with the principles of this invention may be more clearly understood from "ai detailed `consideration oi the drawingsin which:

Figure `lillustratestthe application of the principles of this invention to `a Hartley oscillator wherein the cathode isA maintainedeat R. F. potential With respect tosground;

Figure -2jis-a vector diagrarnfthat is useful in explainingthe operation oi this invention as applied to a Hartleyoscillator;

Figure iillustrates the application of the principles of this invention Ito 'a Hartley oscillator wherein the'cathodeof the oscillator tube is at ground potential;

-Figure4 illustrates the application of the principles of this-'invention to aColpitts oscillator;

1Figure5 isa vector `diagram useful in the explanation offthe operationof the embodiment of the invention showninFigure 4;

Eigured'illustrates another way `in which the principles of'this linvention lnaylbe applied to a Colpitts. oscillator;

-Figure 7 -isfa vector'diagram useful in the eX- planation of the operationof the embodiment of the inventionshown in Figure 6';

Figure f8 ishovvsranother way of applying this inventionitoa-Hartleycoscillator;

Figure gillustrates one-variation inthe application of rthis invention toa Hartley oscillator; and

.Figure lillust-rates the application of this invention: tofa type loi Voscillator employingI a ticltier coil.

yin Figurejl theresfshowna standard Hartley oscillator comprised of a first amplifier generally indicated by the number 2 having a plate il, a grid E and a cathode ii. A parallel resonant circuit iii is connected between the plate 4 and the grid d. The lower end of the parallel resonant circuit is coupled to the grid 6 Via a coupling condenser 52 and a grid leak resistor le that is connected between the grid b and the cathode 8. A B+ potential of suitable magnitude is applied to the plate i through an isolating resistor I6 and a tap it that is at an intermediate point of the inductive branch or the tank circuit lli.

The cathode d is generally coupled to the intermediate point li). In the particular arrangement of Figure 1, the coupling Would normally be accomplished by a condenser connected directly between the cathode 8 and the intermediate point lil so as to prevent the B+ voltage from being grounded.

In accordance with accepted practice, a second amplifier 2d having at least a plate 30, a grid 32 and a common cathode 8 is connected in parallel with at least a portion of the resonant circuit iii. In this particular arrangement the plate 3u of the second amplifier 2S is connected to the plate l of the first amplifier 2. The voltage wave that is to be used for controlling the frequency of the oscillator is supplied by a source E@ via an isolating resistor 33 to the grid 32.

In accordance with this invention, however, the connection between the cathode B and the intermediate point in the tank circuit i8 includes a variable inductance 2t in series with the bypass condenser 2t. In addition, the grid d' is coupled to the cathode 8 by a reactive irnpedance which in this particular figure is shown to be a condenser 2d. The junction of the inductance 2b and the condenser 20 is grounded.

The grid 32 coupled to the junction of the inductance 2t and the bypass condenser 20 via a condenser lill.

The operation of Figure 1 will now be explained in connection with Figure 2 wherein the vector E22 represents the voltage applied between the grid il the cathode il of the oscillator amplifier i. Due to the phase inversion of the normal amplier the voltage appearing at the plate d may be represented by the vector -ulilga It has been theoretically and practically determined that the current owing through the amplifier tube cli a Hartley oscillator lags the voltage across the tank circuit. This current is indicated therefore by the lagging vector IR. The current flowing to the cathode il via the condenser 2G that was connected between the cathode 8 and the grid ii in accordance with this invention is represented by a vector Ic. It will be noted that this vector leads the voltage applied between the grid il and the cathode 3, Ese, by 90. The amount of current reaching the cathode i! via the grid leak resistor i Il is negligible. Applying Kirchoifs law, the current IL iiowing to the cathode 8 via the variable iriductance 2t may be determined by subtracting Ic from the total or resultant current IR. inasmuch as the reactance tube 2B is cathode coupled to the oscillator amplifier 2, the voltage E51. appearing across the inductance 26 is applied so as to modulate the current flowing in the reactance tube 28. This voltage Egi leads the current through the inductance by an angle 0 that is nearly 90 as indicated. Generally, the angle 0 is less than 90 due to the fact that most inductances have a resistive component. With proper selection of the values of the inductance 26 and the condenser 24 that were added to the standard reactance tube Hartley oscillator combination in accordance with this invention, the current drawn by the reactance tube 2B can be made precisely 90 out of phase with respect to the current drawn by the oscillator through the tank circuit lil.

Although the relationship is preferable, this arrangement makes it possible to adjust the angle within wide limits. In the circuits normally used for feeding back the oscillator voltage to the reactance tube, a 90 relationship between the voltage across the reactance tube and the voltage across the tank circuit can only be approached from one side. Actually, as pointed out above, it cannot actually be realized. The fact that this angle can be easily adjusted on either side of 90 makes it easy to adjust the angle at precisely 90. Therefore, the reactance tube can be easily made to act as a pure reactance having no resistive component. In this particular arrangement the reactance tube acts as an inductance. Accordingly, no amplitude variations will appear in the output of the oscillator due to variations in the magnitude of the control voltage wave applied to the grid 32 of the reactance tube 28 from the source Sli.

The voltage applied to the grid of the reactance tube is not reduced by the voltage divider ac tion, as a larger portion of the current drawn by the oscillator passes through the inductance 2d.

Figure 3 illustrates the application of the principles of this invention to a Hartley oscillator wherein the cathode is grounded. This type of circuit is sometimes used to prevent hum from being introduced into the oscillator from the heater elements. However, no change in operation is introduced. For purposes of convenience, these components corresponding to the components of Figure l are indicated by the same numerals.

The principles of this invention may also be applied to a Colpitts oscillator as shown in Fig'- ure 4 and explained in Figure 5. For purposes of simplicity, those components corresponding to components of Figures l and 3 will be indicated by the same numeral.

Figure 4 is similar to Figure 3 in that theY cathode of the reactance and oscillator tubes are grounded. The only essential difference between the Colpitts type oscillator and the Hartley type oscillator is that the common cathode 8 olf the reactance tube and the oscillator is coupled to an intermediate point on the capacitive branch of the tank circuit it. This necessitates the use of two capacitors instead of one, as is well known to those skilled in the art. As has been well established by theory and practice, the current drawn by the Colpitts type oscillator generally leads its plate voltage. However, in this case we have inductance 25 which may alter the phase in the lagging direction so the resultant may be either leading, in phase, or lagging. This is indicated in the vector diagram of Figure 5 by the position of the resultant current vector Ia with respect to the plate Voltage Vector --uEga The current drawn through the condenser 2d to the ycathode 3 of the oscillator tube 2 is indicated by the vector Ic. subtracting this from the vector IR gives the current IL through the inductance 26. The voltage En across the inductance 26 is applied to the grid 32 of the reactance tube by a coupling condenser d. The values 0f the condller 24 and the inductance 26 have been chosen `:soathat the :currents Ici andi-IL iare frelated in such :manner that the @voltage En :applied to the grid of Ithesreactanceitube leads the avoltage=atnthe plate *of 4tthe oscillator tube -e/iEgz by more :than 490?. Thisziisrdonesfor purposesioff :i11- lustrating r the -amount Lof control Aoirerths .phase relationship ithat `may zbe f attained xby using .the principles of this invention.

'l'n ith'e embodiments of the invention previously discussedgarportionfof theaplate currentfof fthe oscillator amplifier 2 iis fpassed athrough ithe condenser 1521i and @the vremainder Lis .passed through the .inductance 2.6. JInxthe :embodiment of the invention shown in Figure I(Lilo-Wever, 'the `condenser2d; and the variablein'ductanee 215 are connected in .series fbetween an ,intermediate point in `'thefcapacitive:branch-of the resonant circuit of sa Colpitts oscillator Vand the cathcdeof `the oscillator ampliiier 5:2. tismctiabelieved necessarlyfto r again describe the 4vstandard components citheiColpitts oscillator, astheyfare''the same -as those `shown 4in Figure "4. 'T-For purposes i of foonvenience, however, corresponding components haveibeenindicated'by-the same numerals.

There is, however, a distinct advantage to the arrangement'oi Figure Ab. `It-will "be noted that the condenseriZidandinductanceZrare connected inseries. Therefore, Vif `they are tuned to resonate near the oscillator frequency alarge increase in voltage may beideveloped across the inductanceZ and applied'to thefgrid 320i the reactance tube. This greatly 4increases fthe-sensitivity-of A.the frequency control. The usevofthe condenser 2t as shown in Figure 6 makes the design of the circuit somewhat simpler, but it can be eliminated in a Colpitts circuit such as that shown. The reason for this is that the B+ voltage is not shunted to ground due to the tuning condensers on either side of the intermediate point I3. The resonant action noted above can still be obtained by suitably choosing the values of the tuned circuit and the value of the variable inductance 26.

The vector diagram of Figure '7 relates to Fig. 6. It needs little explanation as the nomenclature is the same as the vector diagrams previously discussed. The resultant current drawn by the oscillator amplifier 2 is again represented by IP.. The circuit of Figure 6 is a Colpitts type oscillator TP. leads the plate voltage represented by the vector *aEga In this embodiment, however, the current IR ows through both the condenser 24 and the inductance 25. The voltage across the inductance 2E leads this current by an angle 0 and is represented by the vector Egi. This Voltage Egi is applied to the grid 32 of the reactance tube 28 and, as can be seen from the diagram of Figure 6, is 90 away from the oscillator grid voltage Egg. In this circuit no consideration need be given in the current flowing through the condenser 24, as it is identical with the current flowing through inductance 2t. As the inductance 26 is varied so that the combination of the condenser M and the inductance 25 passes from one side of resonance to the other, the angle 6 between the vector IP. and Egi may be varied by large amounts. Even though the Value of the inductance 26 is not such as to be precisely at series resonance with the condenser 24, it can be closely approximated and still have a large amount of control over the angle 6.

Figure 8 illustrates the application of the principles of the invention to a Hartley type oscillator wherein the reactive circuit elements are connected in series as was done in the arrangement ofnEigures. iHerefthe condenser 241is connected between: an intermediate point `on `the inductive branch of the resonant circuit `ltandthe -grid 32 ofithereactanceitube 28. Theinductance26 is connected-between'the-grid:e2 and the `common cathode :8. f'I-hefcondenseri merely serves to bypass .thefRF energy from the oscillator around the source :ci control voltage 355. The vector diagram explainingthe operation Yoi' the` circuit ci Figure would lbefsimilarto'that oi Figure 7 with the: exceptionithatxthe 1 resultant current Ia drawn byitheoscillator :amplier `2 would lag'the plate volt ageivector i [.Ega

Various iother rcircuit `arrangements may be derived'frcm the basic concept oi? this invention. For example, in the Hartley-'oscillator circuit of .FigurerSfthevinductance ispconnected inthe B+ lead. ,The.: B-l-;side=oftheinductance.2t is connected to RF `ground `by `a suitable bypass `ccndenser 142. JThe voltageatthe other end of'the inductance-2S `is `coupled to thegrid 32 of `thereactance'tube 2li-.via coupling condenser is, Aportion'of the.oscillatorcurrent'is bypassed tothe commonrcathodei bythe condenserzi. The operation `of the circuit of Figure 9 is therefore similar to any of the Hartley `type circuits discussedfabove. The onlydifference is in the positionofthercoil.

In all of the arrangements discussed-above `the currentitl'u'cughzthe reactance tube "has been leading theyo'ltage acrossltheirescnant circuit IB f'the'oscillator. Therefore, the reactance tube simulates a 'capacitance connected `across the resonant circuit .l`0. By Tinterchanging the `condenser 24 and the inductance 2t, it is possible to make the reactance tube 28 draw current that lags the Voltage across the resonant circuit lll. Thus, the reactance tube can be made to act as an inductance in parallel with the resonant circuit ll). In doing this, however, the magnitudes of the impedances of the condenser 2t and the inductance 26 should be interchanged.

Figure 10 illustrates the application ofthe principles of this invention to the type of oscillator employing tickler coils. This arrangement is useful where variations inoscillator amplitude as a function of the value of the control voltage employed can be tolerated. For the sake of convenience. components having the same functional relationship as those discussed in the previous figures will be indicated by the same numerals.

It is well known to those skilled in the art that the tank circuit I0 in this type of oscillator may be coupled between the grid '5 of the oscillator tube 2 and the common cathode 8. A tickler coil 50 that is inductively coupled to the inductance of the resonant circuit l0 is connected between the cathode 8 and the plate of the oscillator tube 2. The voltage developed across the tickler coil 5l) is coupled to the grid 32 of the reactance tube 28 Via coupling condenser 52.

Having thus described my invention, what is claimed is:

1. An apparatus for controlling the freouency of an oscillator comprising in combination a Iirst amplier having at least a plate, a grid and a cathode, a resonant circuit connected between said grid and said plate, a rst reactive element coupled between said grid and said cathode, a second reactive element coupled between an intermediate point in said resonant circuit and said cathode, a second amplier having a grid, said second amplifier being connected in parallel with at least a portion of said resonant circuit, means for changing the gain of said second amplifier, and means for applying the voltage across said second reactive element to the grid of said second amplifier.

2. Frequency control apparatus comprising in combination a rst amplifier having a plate, a control grid and a cathode, a parallel resonant circuit coupled between said plate and said grid, a reactive impedance connected between an intermediate point of said tank and said cathode, a different type of reactive impedance coupled between said grid and said cathode, a reactance tube having a control electrode the reactance tube being connected across at least a portion of said parallel resonant circuit and means for coupling said intermediate point to the control electrode of said reactance tube.

3. Apparatus for controlling the frequency of an oscillator comprising in combination a rst emplirler having at least a plate, a cathode and a grid, a parallel resonant circuit connected between said grid and said plate, a capacitance and an inductance connected in series between an intermediate point in said resonant circuit and said cathode, a second amplier connected in parallel with at least a portion of said resonant circuit, a grid associated with said second amplifier, and coupling circuits for applying the voltage appearing across said induotance to said latter grid.

4. Apparatus for controliing the frequency of an oscillator comprising in combination a rst ampliiier having at least a plate, a cathode and a grid, a parallel resonant circuit connected between said grid and said plate, a capacitance and an inductance connected in series between an intermediate point in said resonant circuit and said cathode, a second amplier connected in parallel Wtih at least a portion of said resonant circuit, a grid associated with said second amplier, and coupling circuits for applying the Voltage appearing across said capacitance to said latter grid.

5. An oscillator, a resonant circuit coupled to said oscillator so as to control its frequency, a reactance tube, said reactance tube having electrodes for controlling its conduction, a condenser and an inductance connected in series, the values of said condenser and inductance 'being such that they are in series resonance at the frequency of the oscillator, said series combination being ccnnected between an intermediate point of said resonant circuits and one of said electrodes, and means for coupling a point between said inductance and condenser to another control electrode so that the voltage across one half said series combination controls the conduction of v said reactance tube.

EUGENE O. KEZER.

References Cited in the file of this patent UNITED STATES PATENTS 

